Guidewires are well known for placing and guiding catheters and other devices in lumens of the human body, such as in the vascular network. In a common type of procedure, a guidewire is inserted percutaneously into an easily accessed blood vessel. The guidewire then is manipulated to steer the guidewire through the vascular network until the distal end (the end inside the patient) reaches a desired location. The catheter may be inserted preassembled with the guidewire or the catheter may be inserted and advanced over a previously placed guidewire.
The performance of a guidewire is influenced by certain characteristics such as steerability, kink-resistance, flexibility and stiffness. The steerability of the guidewire is important especially when a tortuous path must be navigated to reach the target site as is commonly encountered when placing a catheter, such as an angioplasty catheter, in the coronary arteries. Steering is executed from the proximal end of the guidewire (outside of the patient) by rotating, pushing and pulling on the guidewire to cause corresponding movement at the distal tip of the wire. The distal tip typically is slightly bent so that when rotated the tip can be directed toward a selected one of several vascular branches. The distal tip of the guidewire commonly is radiopaque so that its movement can be observed under x-ray fluoroscopy.
Kinking is the result of a plastic deformation of the guidewire and usually is characterized by a sharp deformation or point bend of the distal section of the wire. Such a deformation may result from attempting to pass a guidewire through a relatively hard, calcified lesion, a mostly occluded vessel section or a very tortuous vascular section. The distal end of the wire may kink or bend back upon itself in a condition referred to as prolapse. Thereafter, the wire may return to its original shape, or it may remain permanently deformed if, during the bending, the wire material is stressed beyond its elastic limit.
Once kinked, the guidewire loses its controllability and usually must be discarded because the physician cannot adequately straighten the wire to remove the kink. Consequently, the procedure may have to be aborted and a new guidewire selected, reinserted, and again manipulated and advanced to the target site. Reinsertion of another guidewire increases the risk of trauma to the blood vessels and adds to the time necessary to complete the procedure. Additionally, because placement of the guidewire typically is done under X-ray fluoroscopy, the reinsertion procedure will require that the patient be exposed to additional fluoroscopic radiation.
Guidewires typically involve a balance between flexibility and stiffness characteristics. It is important that the guidewire be sufficiently flexible, particularly at its distal region, so that it does not damage the wall of the blood vessel and so that it can adapt itself to the path of the blood vessel into which it is being inserted. A stiff guidewire, however is preferred for a variety of reasons. A stiff guidewire generally provides better response of the distal end to torsional and longitudinal manipulation from the proximal end. A stiff guidewire promotes proper alignment and integrity of the catheter in relation to the blood vessel wall. A stiff guidewire is less likely to be pulled out of position in the blood vessels when the catheter is advanced over the guidewire, a characteristic referred to as "trackability".
Attempts have been made to improve the steerability, kink-resistance, flexibility and/or stiffness of medical guidewires. Often, the material or structural modifications that have enhanced a particular guidewire characteristic have adversely affected one or more of the other guidewire properties. Consequently, guidewire construction typically has involved trade-offs and compromises between the aforementioned characteristics. Guidewires historically have been made from fine stainless steel monofilament. Among the various guidewire constructions typical of the prior art are guidewires that include an elongate helical coil having an internal core wire attached at its ends to the ends of the helical coil. In another type of guidewire, such as a small diameter steerable guidewire of the type described in U.S. Pat. No. 4,545,390 (Leafy), the guidewire includes a proximal shaft formed from a solid wire having a distal tapered portion and a helical coil mounted about the distal tapered portion. Although a reduction in the diameter of the wire used to make the guidewire tends to increase flexibility it also tends to decrease stiffness with consequent reduction in manipulative and steering control. Although improvements in guidewire construction have been made, the ideal characteristics of a guidewire still requires a balancing of competing characteristics.
Psuedoelastic alloys such as Nitinol have also been used to form guidewires. Nitinol guidewires exhibit exceptional flexibility at the temperature encountered in the human body but are not easily steered because the distal tip of the guidewire cannot retain a shaped bend.
Guidewires also have been made out of plastic. While plastic guidewires tend to be more flexible than stainless steel, particularly at larger diameters, they are less kink resistant. Plastic guidewires, when packaged in the typical rolled-up coils, also suffer from permanent plastic deformation when stored in the coil over a period of time, a condition referred to as creep.
Guidewires also have been formed from combinations of metal and plastic. A thin layer or covering of a polymer, such as Teflon, has been applied to a stainless steel wire core to improve the lubricity of the guidewire. The Teflon surface coating has little appreciable affect on the kink-resistance, flexibility, steerability or stiffness of the metal core. An additional example of a guidewire formed from a combination of metal and plastic is that described in U.S. Pat. No. 3,789,841 Antoshkiw in which the guidewire is provided with a core wire surrounded by an outer plastic jacket.